The invention relates generally to wireless communications. More particularly, the invention relates to generating multiple processed symbol streams from a single incoming symbol stream to provide transmission diversity through multiple transmitters, receiving the multiple transmitted processed symbol streams and extracting the original single incoming symbol stream.
Wireless communication systems commonly include information carrying modulated carrier signals that are wirelessly transmitted from a transmission source to one or more receivers within an area or region. FIG. 1 shows modulated carrier signals traveling from a transmitter 110 to a receiver 120 following many different (multiple) transmission paths.
Multipath is the composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and receiver. The receiver may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the receiver later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal is a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several microseconds.
The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. Fading causes the modulated carrier signal to attenuate in amplitude when multiple paths subtractively combine.
In line-of-sight (LOS) wireless systems, multipath is usually minor and the effects can be minimized. The amplitudes of the reflected signals are generally much smaller than the primary signal and can be effectively filtered out using standard techniques. However, in non-line-of-sight (NLOS) wireless systems, the reflected signals may have greater relative power levels because the primary signal may be partially or totally obstructed, and typically more multipath is present.
Transmitter diversity is a technique used in multiple antenna-based communication system to reduce the effects of multi-path fading. Transmitter diversity can be obtained by providing a transmitter with two or more (N) antennas. These N antennas imply N channels that suffer from fading in a statistically independent manner. Therefore, when one channel is fading due to the destructive effects of multi-path interference, another of the channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent channels, a receiver can often reduce the detrimental effects of fading.
While prior art transmitter diversity systems mitigate the effects of fading, prior art transmitter diversity systems are generally susceptible to narrow band frequency interference. Additionally, many prior art transmitter diversity systems require channel knowledge at the transmitter.
It is desirable to have an apparatus and method that provides a diversity transmitter system for wirelessly transmitting information that results in minimal loss of the transmitted information due to fading and multipath. It is desirable that the transmitter diversity system spreads information in time/frequency/space dimensions, and be immune to narrow band frequency interference. Additionally, it is desirable that the transmitter diversity system easily allows for additional coding redundancy.
The invention includes an apparatus and a method for generating a plurality of processed transmission symbol streams based on an incoming symbol stream, and transmitting the processed transmission symbol streams over a plurality of transmitters. The invention further includes receiving the processed transmission symbol streams, and extracting the original pre-transmission symbol stream. The processing mitigates the effects of fading and multi-path. The processing also provides spreading of information in frequency and space, and includes immunity to narrow band frequency interference.
A first embodiment of the invention includes a diversity transmission system. The diversity transmission system includes a diversity transmitter receiving incoming symbols. The diversity transmitter includes at least one transmitter antenna transmitting a plurality of multi-carrier modulated signals. Each multi-carrier-modulated signal includes a corresponding processed symbol sub-block stream. Each symbol of the processed symbol sub-block stream is based on a linear transform of a plurality of incoming symbols. The diversity transmission system further includes a diversity receiver. The diversity receiver includes at least one receiver antenna receiving the plurality of multi-carrier modulated signals after the multi-carrier modulated signals having been modified by transmission channels between the transmitter antennas and the receiver antenna. The diversity receiver further includes at least one multi-carrier demodulator for demodulating the received multi-carrier modulated signals and generating a demodulated symbol stream. A symbol-processing unit receives the demodulated symbol stream and generates a stream of outgoing symbols.
A second embodiment of the invention includes a diversity transmitter. The diversity transmitter includes a symbol-processing unit for receiving a stream of incoming symbols and generating a plurality of processed symbol streams. The processed symbol streams are based on linear transforms of the incoming symbols. A plurality of multi-carrier modulators generate a plurality of multiple-carrier modulated signals. Each multi-carrier modulator receives a corresponding processed symbol stream and generates a multi-carrier-modulated signal based on the corresponding processed symbol stream.
A third embodiment is similar to the second embodiment. The third embodiment includes at least two distinct channels that are formed by a plurality of antennas within an antenna array.
A fourth embodiment is also similar to the second embodiment. The fourth embodiment includes at least two distinct channels that are formed by a plurality of antennas having different polarization states.
A fifth embodiment is also similar to the second embodiment. The fifth embodiment includes at least two distinct channels that are formed by a plurality of separately located transmitters.
A sixth embodiment is similar to the second embodiment. The sixth embodiment further includes the processed streams being generated by multiplying blocks of the incoming symbols with pre-stored linear transform matrices.
A seventh embodiment includes a diversity receiver. The diversity receiver includes at least one receiver antenna receiving a plurality of multi-carrier modulated signals. Each carrier signal of each multi-carrier-modulated signal is modulated by a processed symbol that includes a linear transform of a plurality of incoming symbols to a transmitter. The diversity receiver further includes at least one multi-carrier demodulator for receiving and demodulating the transmission channel modified multi-carrier modulated signals and generating a demodulated symbol stream. A symbol-processing unit receives the demodulated symbol stream and generates a stream of outgoing symbols.
An eighth embodiment includes a method of diversity transmitting. The method includes receiving a stream of incoming symbols and generating a plurality of processed symbol streams. The processed symbol streams are based on linear transforms of the incoming symbols. The method further includes generating a plurality of multiple-carrier modulated signals. Each multiple-carrier-modulated signal is based on a corresponding processed symbol stream.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.